Re: [PATCH v6 3/3] mm/gup: disallow FOLL_LONGTERM GUP-fast writing to file-backed mappings

[Lorenzo Stoakes]

On Tue, May 02, 2023 at 05:09:06PM +0200, David Hildenbrand wrote:
> On 02.05.23 15:56, Matthew Rosato wrote:
> > On 5/2/23 9:50 AM, Jason Gunthorpe wrote:
> > > On Tue, May 02, 2023 at 03:47:43PM +0200, David Hildenbrand wrote:
> > > > > Eventually we want to implement a mechanism where we can dynamically pin in response to RPCIT.
> > > >
> > > > Okay, so IIRC we'll fail starting the domain early, that's good. And if we
> > > > pin all guest memory (instead of small pieces dynamically), there is little
> > > > existing use for file-backed RAM in such zPCI configurations (because memory
> > > > cannot be reclaimed either way if it's all pinned), so likely there are no
> > > > real existing users.
> > >
> > > Right, this is VFIO, the physical HW can't tolerate not having pinned
> > > memory, so something somewhere is always pinning it.
> >
> > I might have mis-explained above.
> >
> > With iommufd nesting, we will pin everything upfront as a starting point.
> >
> > The current usage of vfio type1 iommu for s390 does not pin the entirety of guest memory upfront, it happens as guest RPCITs occur / type1 mappings are made.
>
> ... so, after the domain started successfully on the libvirt/QEMU side ? :/
>
> It would be great to confirm that. There might be a BUG in patch #2 (see my
> reply to patch #2) that might not allow you to reproduce it right now.
>

Yes apologies - thank you VERY much for doing this Matthew, but apologies, I
made rather a clanger in patch 2 which would mean fast patch degrading to slow
path would pass even for file-backed.

Will respin a v7 + cc you on that, if you could be so kind as to test that?

> --
> Thanks,
>
> David / dhildenb
>